Semiconductor memories, as first developed, were inherently volatile (i.e., would not retain the stored data if power was removed), and various methods have been developed to either make the devices nonvolatile or to add backup batteries to the circuitry attached to the integrated circuit. In particular, one method of adding backup batteries has been to package one or two lithium batteries in the same housing with an otherwise volatile semiconductor memory to make the memory operate as a nonvolatile memory. Small lithium batteries have been developed in the last few years, and accelerated life tests have indicated that, under a relatively light electrical load, the battery should provide voltage sufficient to retain data in a semiconductor memory for well over ten years if the battery is operated at room temperature.
Such nonvolatile data storage makes it possible to utilize a transportable electronic data storage module which can be connected to an external circuit and be written into, be disconnected from the external circuit with no power applied to the module, and later be reconnected to the same or another external circuit, and have its data read out.
However, it has been discovered that this process is unreliable in that some of the data is lost on an intermittent basis, and also, at times, the life of the lithium batteries has been severely shortened.
It can therefore be appreciated that a method or apparatus to make this transportable data storage process reliable and which also prevents severe degradation of the lithium batteries would be highly desirable.